Alloy steel article and method of producing



1967 M. BERNICK ETAL 3,

ALLOY STEEL ARTICLE AND METHOD OF PRODUCING Filed NOV. 29, 1963 r; in

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United States Patent 3,303,064 ALLGY STEEL ARTICLE AND METHOD 0F PRODUCING Leslie M. Berniclr, Calumet City, and Melvin B. Gibbs,

Harvey, ill, and Carlton F. Schrader, Chesterton, Ind.,

assignors to Inland Steel Company, (Ihicago, 11]., a

corporation of Delaware Filed Nov. 29, 1963, Ser. No. 327,011 Claims. (Cl. 148-16) The present invention relates generally to an alloy steel article free of objectionable surface markings which are normally formed therein during cold rolling, and more particularly to an improved cold reduced low-carbon killed alloy steel sheet material which does not have 0' jectionable surface markings thereon and to the method of producing an improved steel sheet material and improved vitreous enameled article therefrom.

Heretofore it has been found possible to produce a steel article, such as a cold reduced low-carbon steel sheet material which is relatively non-aging and which has good drawing properties and good vitreous enameling properties by incorporating in the steel melt a quantity of titanium, preferably in an amount sufiicient to combine with the uncombined carbon, nitrogen and oxygen in the steel melt (see Comstock et al. US. Patent No. 2,495,835). The foregoing cold reduced low-carbon steel sheet material containing titanium, after coating with a single coat of light-colored vitreous enamel, is relatively free of many defects commonly found in single coat white enameled articles where no dark-colored ground coat is used. A serious defect is encountered, however, as a result of incorporating titanium in the steel melt; namely, the formation of longitudinal lines which are called steel lines or shadow lines on the surface of the cold reduced article or vitreous enameled steel strip, and these steel lines are still clearly visible as shadow lines after applying the single light-colored coating of vitreous enamel.

It is therefore an object of the present invention to provide a low-carbon steel containing an alloying element, such as titanium, as a cold reduced article which does not exhibit objectionable surface markings.

It is a further object of the present invention to provide an improved titanium-containing col-d reduced enameling steel material which is free of steel lines.

It is still another object of the present invention to provide an improved single coat light-colored vitreous enameled cold reduced low-car bou killed steel sheet material containing titanium which does not exhibit shadow lines.

It is a further object of the present invention to provide a method of making a cold reduced low-carbon steel article containing titanium which does not exhibit steel lines or shadow lines.

It is also an object of the present invention to provide a method of enameling a cold reduced low-carbon killed steel sheet material containing titanium with a single coat of light-colored vitreous enamel which avoids forming shadow lines.

Other objects of the present invention will be apparent to those skilled in the art from the following detailed description and claims when read in conjunction with the accompanying drawing wherein;

FIG. 1 is a schematic plan view of a section of cold reduced alloy steel which has been treated in accordance with standard operating procedure and which exhibits typical steel lines on the surface thereof;

FIG. 2 is a schematic plan view of the section of steel of FIG. 1 which has a conventional single coat white vitreous enamel coating thereon and which exhibits Patented Feb. 7, 1967 shadow lines corresponding to the heavy steel lines in the section of steel; and

FIG. 3 is a schematic plan view and section of cold reduced titanium alloy steel having a conventional single coat white vitreous enamel coating thereon which has been processed in accordance with the present invention and which is free of steel lines and shadow lines.

It has heretofore been found that a low-carbon steel or ingot iron which is relatively non-aging in all conditions of heat treatment and which can be deep drawn, rolled and otherwise extensively cold-worked for many uses, can be made by incorporating titanium in the steel melt in an amount suflicient to combine with all the carbon present, or somewhat in excess of this amount. The amount of titanium required to combine with the carbon to form titanium carbide is approximately four times the amount of carbon in the steel. However, to account for usual losses incident to practical operation, addition to the melted steel of 4.5 to 5 times as much titanium as carbon has been found to be advisable, although titanium in an amount 30 times the carbon content has been used. Generally, the titanium content of the finished steel is well below 1%, and incorporation of titanium in quantities greater than this merely increases the expense of the operation without obtaining compensating or attendant advantages. In general, the steel stock should be a relatively low-carbon steel in which the carbon is not in excess of about 0.15%, and preferably below about .05 C. The steel or ingot iron produced in accordance with the present invention and containing carbon not substantially in excess of 0.15% and containing at least 4.5 times as much titanium as carbon is free from any kind of strain-aging, possesses exceptionally high ductility and shows no definite or sudden yield point when stressed above the amount required for the occurrence of plastic deformation. Thus, the alloy steel of the type used in the present invention in sheet form can be cold-drawn without tearing or wrinkling more severely than regular black plate, that is to say, lowcarbon steels containing insuflicient titanium or other similar alloying element to combine with all the carbon, and thus possesses qualities making this stock a superior base for use where good drawing properties are required, including the manufacture of articles to be vitreous enameled.

By employing a titanium-containing steel of the foregoing type as vitreous enameling base stock, a single enamel coating of various kinds, including standard white cover-coats, can be fused to the metal surface without the necessity of employing the usual ground-coat, and the resultant enamel coating is free from blistering and reboiling defects. These titanium-containing alloy steels and similar steels containing equivalent alloying elements are particularly good bases for use in the production of vitreous enameled articles, such as panels for household appliances and the like.

The addition of titanium in the above indicated amounts to a low-carbon steel melt, however, produces in the cold rolled or drawn steel sheets or strips highly objectionable steel lines or shadow lines which appear as irregularly spaced dark lines of varying width and length formed in the steel surface and extending longitudinally thereof. The steel lines or shadow lines appear to be the result of the preferential oxidation of certain ingredients in the titanium alloy steel which on cold reduction treatment, such as in the cold rolling treatment required to produce a thin steel strip, creates longitudinally extending lines or markings on the smooth surface of the cold reduced steel (see FIG. 1). These markings or steel lines detract from the quality of the steel sheet and from an enameled product made therefrom, since the markings are plainly visible even after the steel strip is coated with a single coat of light-colored vitreous enamel wherein a dark-colored ground coat is not employed in accordance with the preferred enameling practice (see FIG. 2).

While various unsuccessful attempts have been made to eliminate steel lines or shadow lines from a cold reduced titanium-containing alloy steel, it has now been dis-covered that the objectionable steel lines can be eliminated from the cold reduced titanium alloy steel sheet material and from the single coat light-colored vitreous enameled articles made therefrom by subjecting the preferably clean cold reduced alloy steel material to a special pre-treatment. More particularly, the steel line defect in a cold reduced titanium-containing steel can be eliminated by subjecting the cold reduced or cold drawn steel to a controlled heat treatment in a reducing atmosphere following the cold reduction thereof without previously exposing the said cold reduced steel to a highly oxidizing atmosphere, such as encountered in burning off rolling lubricants. And, if it is desired to use the said heat treated titanium steel for vitreous enameling, it has been further found necessary to subject the heat treated strip in a pre-enameling treatment to a controlled pickling treatment in an acid pickling bath of any conventional composition to effect a critical pickle weight loss between about six tenths of a gram to about two grams per square foot of surface area. The cold reduced steel treated in the foregoing manner, if desired, can then be subjected to a conventional single coat of light-colored vitreous enameling treatment, preferably after applying a conventional nickel flash coating in accordance with good enameling practice.

The temperature and time of heating used in the heat treating step of the present invention can be varied considerably without detracting from the improved results. Generally, the temperature and time of heat treating the cold reduced low-carbon titanium-containing steel in a closed chamber are always adjusted to provide the steel with the desired mechanical properties required. For example, an enameling steel sheet material must have the necessary strain and aging characteristics for deep drawing, and the heat treating temperature is adjusted accordingly. In every instance, however, the atmosphere in the heating chamber is carefully controlled so that the atmosphere in which the steel is heated remains reducing during the heat treating period and does not become oxidizing while the exposed surface of the strip or sheet is at an elevated temperature.

The herein required heat treatment of the cold reduced steel strip can be continuously carried out on an endless steel strip by employing a continuous normalizing or continuous annealing furnace wherein the atmosphere can be maintained in a reducing state. When continuously heat treating a clean steel strip in a continuous normalizer of conventional design the steel strip is heated continuously by passing through a heated zone having a reducing atmosphere, such as dry 10% hydrogen and about 90% nitrogen atmosphere (i.e. HNX gas), with a dew point of the atmosphere preferably ranging between about 30 F. and F. and wherein the strip is heated to a temperature of about 1650 F. for a few seconds, preferably about two seconds. The strip is preferably cleaned with an alkaline aqueous solution or other solution which does not oxidize the surface thereof. The foregoing heat treatment, in addition to having the desired beneficial effect on the shadow line defect, also imparts the desired mechanical properties to the alloy steel strip which permits a substantial degree of elongation without adversely affecting the subsequent appearance thereof in exposed areas or loss of structural strength.

The same beneficial effects on the steel lines and the mechanical properties of the steel strip can be achieved by heat treating the cold reduced titanium-containing steel strip batchwise in a high temperature open coil boxannealing furnace. In the open coil heat treatment, the steel 4 strip is first placed in an open coil form in the conventional manner to provide spaces betwen turns on the coil for the passage of gas therebetween and slowly heated in a reducing atmosphere preferably to a temperature of about 1620 F. over a period of between 2 and 4 hours. The coil is held at this soaking temperature of about 1620 F. for a period of between 2 and 3 hours to insure that the entire coil is evenly heated and is then heated to a temperature of between about 1675 F. to 1695 F. The coil is held at the latter temperature in accordance with standard open coil annealing procedure for between about 30 minutes and one hour to attain the desired physical or metallurgical properties in the strip. The reducing atmosphere maintained in the open coil annealing furnace, which is preferably a hydrogen-nitrogen gas but which can be dissociated ammonia or other reducing 7 gas, is relatively dry and has a sufficiently low dew point to remain non-oxidizing. Particularly during the initial heating up period when the temperature is relatively low, the dew point of the furnace atmosphere is kept low and is preferably maintained not greater than about +4 F. Thereafter, the dew point of the furnace can vary as high as +40 F. or above, provided the temperature is sulficiently elevated so that the furnace atmosphere is nonoxidizing and preferably remains reducing.

Before the heat treated titanium-containing steel strip can be used for single coat light-colored vitreous enameling, the strip should be pickled immediately before theenameling treatment by immersing in an acid pickling bath until a critical pickle weight loss is effected in order to obtain a satisfactory single coat light-colored vitreous enamel product. It has been found necessary to maintain the heat treated steel strip or article in an acid pickling bath until the pickle loss is at least about .6 gram per square foot and not greater than about 2 grams per square foot. The preferred amount of pickle loss effected ranges between about .75 gram per square foot and 2.0 grams per square foot. Within the latter range of pickle loss a single coat light-colored vitreous enameled strip made from the heat treated pickled strip exhibits excellent enamel adherence and is free of steel lines or shadow lines (see FIG. 3).

Any conventional acid pickle bath, such as an aqueous sulfuric acid solution containing between about 6% to 10% by weight sulfuric acid, can be used to effect the required pickle loss. It is also possible to employ other acid pickling baths, if desired.

The following specific examples. are given in order to further illustrate the present invention without, however, limitingthe invention to the specific materials and conditions used.

EXAMPLE 1 A low-carbon titanium-containing steel prepared in accordance with the procedure described in US. Patent No. 2,495,835 and having the following chemical analysis: C, .045; Mn, .30; P, .010; S, .019; Si, .03; A1, .018; Ti, .34; which has been cold reduced to form a thin strip having a thickness of about .0449 inch and formed into an open coil weighing about 10,960 pounds was heat treated in an open coil annealing furnace wherein the temperature was raised to 1620 F. over a period of 4 hours and held at 1 62 0 F. for 2 hours. The furnace atmosphere during the heating up period was comprised of 10% hydrogen and about 90% nitrogen \and was relatively dry with a dew point of about 10 F. During the 2-hour holding period the .said atmosphere had a dew point of between about +24 and +30 F. Thereafter the furnace was heated to a temperature between 1670" F. and 1695 F. and held at the latter temperature for 40 minutes with the 1 0% hydrogen-90% nitrogen atmosphere having a dew point of between +17 F. and +19 F. It should be understood that the atmosphere in the heat treatment fur- 5 the strip being exposed to oxidizing conditions before the heat treatment thereof, a phenomenon which appears to have a causative relationship with the formation of shadow lines on the surface of the cold reduced 6 EXAMPLE 4 A plurality of cold-rolled steel panels having the chemi- I cal composition of Example 1 and heat treated as in Example 1 were treated batchwise by a vitreous enameler in titanium-containing alloy steels. The steel str p heat 5 conventi on all commercial pickling equipment by treated in the above manner was free of steel lines or ulf 1 h shadow lines and had good mechanical properties merSm-g m an aqueous S unc acl aVm-g concentration of 7.46% to 7.9% sulfuric acid by weight and EXAMPLE 2 at a temperature of 170 F. Thereafter the enameler ap- The heat treated product of Example 1 was pickled in 10 Phed a z ii fi f g i as m Exampllef g fi a 6% by weight sulfuric acid solution at a temperature of lf' ;g ofore V1 l g i e e about 150 F., followed by a conventional 5-minute nickel g a ig flash treatment by contacting the pickled strip with a nickel Ion manner gwe e resu S Own m a B Table III Time Wt. Loss, Time Ni Wt., Enamel Enamel Appear: nee (Min) gin/it. (Min) gin/ft. Adherence 10 1. 4 No shadow lines. 10 .s 20 .25 Do. 15 1.6 15 .27 Do. 20 1.8 15 .21 Do. 15 .8 12 .14 Do. 15 1.0 12 .16 Do.

solution having a nickel concentration of about 1.6 ounces Others may practice the invention in any of the numerper gallon at a temperature of about 165 F. A convenous ways which are suggested to one skilled in the art tional one-coat white vitreous enamel coating was then by this disclosure, and all such practice of the invention applied in the usual manner. Test strips of the above are considered to be a part hereof which fall within the material having the indirected pickle treatment exhibited scope of the appended claims. the properties shown in Table I: We claim:

Table I 1. In a method of forming a cold reduced alloy steel sheet material which is free of objectionable longitudinally extending surface markings normally appearing in Pickle Time 235,? $333,33 Enamel Appearance the surface thereof after cold reduction and conventional m/[L heat treatment, the improvement which comprises; heating a cold reduced low-carbon titanium containing steel sminutes .60 Fair No shadow line sheet material having objectionable longitudinally extend- :fifigg: 0 :12 ggg 3g: ing surface markings in a reducing non oxidizing atmosphere to a temperature of at least about 1620'" F.

A sample of the said heat treated titanium-containing steel which had been pickled for only 5 minutes effecting a pickle loss of only .27 gram per square foot exhibited after vitreous enameling very poor enamel adherence, as evidenced by the conventional hammer impact test.

EXAMPLE 3 A low-carbon titanium-containing steel of Example 1, in open coil form, was subjected to a heat treatment in an open coil annealing furnace which comprised heating the strip to 162-0 F. over a period of 2.5 hours in an atmosphere of 10% hydrogen and about 90% nitrogen with a dew point between +17 F. and +3 F. The coil was then held at a temperature of 1620 F. for 2.5 hours while the atmosphere had a dew point of about +5 F. The coil was then heated to 1670 F. and held at the latter temperature for one half hour in an atmosphere of the above composition having a dew point of -4 F.

Thereafter the heat treated strip was pickled in a 9% by weight sulfuric acid aqueous solution, nickel flashed as in Example 1, and one-coat white enameled in the con ventional manner to give the test results shown in Table II:

without previous exposure of the said cold reduced steel sheet material to surface oxidation during said heat treatment, the amount of titanium in said steel being at least sufficient to combine with all the carbon in the steel.

2. A method as in claim 1 wherein the said reducing non oxidizing atmosphere consists essentially of hydrogen and nitrogen with a maximum dew point of about +3 0 F.

3. In a method of forming a low-carbon cold reduced titanium-containing steel sheet material which is free of objectionable longitudinally extending surface markings normally appearing in the surface thereof after cold reduction and conventional heat treatment, the improvement which comprises; heating a cold reduced low-carbon titanium-containing steel sheet material having objectionable longitudinally extending surface markings in a reducing non oxidizing atmosphere to a temperature of at least about 1620 F. without previously exposing said color reduced steel sheet material to surface oxidation at a heat treating temperature, the amount of titanium in said steel being at least sufiicient to combine with all the carbon in the steel.

4. A method as in claim 3 wherein the said reducing non oxidizing atmosphere consists essentially of hydrogen and nitrogen with a maximum dew point of about +30 F.

5. In a method of forming a low-carbon cold reduced titanium-containing steel sheet material for vitreous enameling, the improvement which comprises; heating a cold reduced low-carbon titanium-containing steel sheet material having objectionable longitudinally extending surface markings in a reducing non oxidizing atmosphere to a temperature of at least about 1620 F. without previously exposing said cold reduced sheet material to surface oxidation at a heat treating temperature, and pickling said sheet material to effect a pickle weight loss of between about .6 gram per square foot and about 2 grams per square foot, the amount of titanium in said steel being at least sufficient to combine with all the carbon in the steel.

6. In a method of forming a cold reduced alloy steel vitreous enameled sheet material which is free of objectionable longitudinally extending surface markings, the improvement which comprises; heating a cold reduced low-carbon titanium containing steel sheet material having objectionable longitudinally extending surface markings in a reducing non oxidizing atmosphere to a temperature of at least about 1620 F. without previously exposing said cold reduced steel sheet material to surface oxidation at a heat treating temperature, pickling said sheet material to effect a pickle weight loss of be tween about .6 grams per square foot and about 2 grams per square foot, and thereafter applying a single coat light-colored vitreous enamel layer to said pickled sheet material, the amount of titanium in said steel being at least sufficient to combine with all the carbon in the steel.

7. In a method of forming a single-coat light-colored vitreous enameled sheet material free of objectionable longitudinally extending surface markings, the improvement which comprises; heating a cold reduced low-carbon titanium-containing steel sheet material having objectionable longitudinally extending surface markings in a reducing non oxidizing atmosphere at a temperature of at least about 1620 F. without previously exposing said cold reduced steel sheet material to surface oxidation to a heat treating temperature, pickling said sheet material to effect a pickle weight loss of between about .6 gram per square foot and about 2 grams per square foot, and thereafter applying a single coat of light-colored vitreous enamel to said pickled sheet material, the amount of titanium in said steel being at least suflicient to combine with all the carbon in the steel.

8. In a method of forming a single coat light-colored vitreous enameled steel sheet material from a cold reduced titanium-containing steel, the improvement which comprises; heating a cold reduced low-carbon titaniumcontaining steel sheet material having objectionable longitudinally extending surface markings in a reducing non oxidizing atmosphere to a temperature of between about 1620 F. and 1695 F. without previously exposing said cold reduced steel sheet material to surface oxidation at a heat treating temperature, pickling said steel sheet material to efiect a pickle weight loss of between about .6 gram per square foot and 2 grams per square foot, and applying to said pickled sheet material a single coat of light-colored vitreous enamel; whereby said sheet material is provided with a firmly adherent vitreous enamel coating free of visible longitudinally extending surface markings, the amount of titanium in said steel being at least sufficient to combine with all the carbon in the steel.

9. A method as in claim 8 wherein said pickle weight loss is between about .75 gram per square foot and 2.0 grams per square foot.

10. A cold reduced low-carbon titanium-containing allow steel which is free of objectionable longitudinally extending surface markings when produced by the process of claim 5.

References Cited by the Examiner UNITED STATES PATENTS 2,455,331 11/1948 Eckel et al. 14s -12.1 2,495,836 1/1950 Comstock 117-129 2,602,034 7/1952 Eckel 14s 12 2,755,210 7/1956 Sutphen-etal l4816 3,055,779 9/1962 Chu et al. 14816 X 3,193,417 7/1965 Kopchak 117 129 DAVID L. RECK, Primary Examiner. 

1. IN A METHOD OF FORMING A COLD REDUCED ALLOY STEEL SHEET MATERAL WHICH IS FREE OF OBJECTIONABLE LONGITUDINALLY EXTENDING SURFACE MARKINGS NORMALLY APPEARING IN THE SURFACE THEREOF AFTER COLD REDUCTION AND CONVENTIONAL HEAT TREATMENT, THE IMPROVEMENT WHICH COMPRISES; HEATING A COLD REDUCED LOW-CARBON TITANIUM CONTAINING STEEL SHEET MATERIAL HAVING OBJECTIONALBY LONGITUDINALLY EXTENDING SURFACE MARKINGS IN A REDUCING NON OXIDIZING ATMOSPHERE TO A TEMPERATURE OF AT LEAST ABOUT 1620*F. WITHOUT PREVIOUS EXPOSURE OF THE SAID COLD REDUCED STEEL SHEET MATERIAL TO SURFACE OXIDATION DURING SAID HEAT TREATMENT, THE AMOUNT OF TITANIUM IN SAID STEEL BEING AT LEAST SUFFICIENT TO COMBINE WITH ALL THE CARBON IN THE STEEL. 